Simplify the following expression: $5\sqrt{45}$.
Explanation: First, try to factor a perfect square out of the radical. $= 5\sqrt{45}$ $= 5\sqrt{9 \cdot 5}$ Separate the radical and simplify. $= 5\sqrt{9} \cdot \sqrt{5}$ $= 5 \cdot 3 \cdot \sqrt{5}$ $= 15\sqrt{5}$